Instrument damping system



y 17, 1955 w. H. SKIDMORE 2,708,737

INSTRUMENT DAMPING SYSTEM Filed June 18, 1952 WAL Tfl? H. .SK/DMORE IN VEN TOR.

ATT' NEYS United States Patent ti INSTRUMENT DAMPING SYSTEM Walter H.Skidmore, Rutherford, N. J., assignor to Weston Electrical InstrumentCorporation, Newark, N. J., a corporation of New Jersey Application June18, 1952, Serial No. 294,287 2 Claims. (Cl. 324-125) This inventionrelates to damping devices for electrical instruments and moreparticularly to a novel damping device afiording selective control ofthe damping effect upon the instrument movable mechanism.

Damping devices are used to advantage on electrical indicatinginstruments to facilitate the reading of the pointer position relativeto an associated scale. in general, the movable mechanism of theinstrument, having significant inertia, normally oscillates about thefinal steady-state position thereby making a reading of the instrumentindication diflicult in those instances where the deflection forceapplied to the mechanism varies over a range of discreet values infairly rapid sequence. In the case of a direct current indicatinginstrument the damping problem has been solved by winding the movablecoil upon a closed, metallic frame having a relatively low electricalresistance. in alternating current instruments, such as the movableiron, induction and electro-dynamometer types, the problem of providingsufiicient damping of the movable mechanism is somewhat more complicatedespecially when such factors as minimum overall instrument size, andselective adjustability of the damping factor, are taken intoconsideration.

An object of this invention is the provision of a magnetic dampingdevice for an electrical instrument and including selectively-operablemeans external of the instrument effective to alter the dampinginfluence of the device on the instrument movable mechanism.

An object of this invention is the provision of a damping device for anelectrical instrument said device being of the permanent magnet andelectro-magnet type whereby various degrees of damping or over-dampingof the movable mechanism may be obtained.

An object of this invention is the provision of a damping device for anelectrical mechanism having a movable vane, said device comprising apair of spaced, magnetic material plates, alined permanent magnetscarried by one of the plates and having polar surfaces spaced arelatively small distance from the vane, and electromagnets carried byone of the plates, said electro-magnets including a magnetic materialcore having one end in contact with the supporting plate and the otherend spaced from the said vane.

These and other objects and advantages will become apparent from thefollowing description when taken with the accompanying drawings inwhich:

Figure l is a top plan view of an instrument movable mechanismcomprising a pivotally-mounted coil carrying a pointer and damping vane;

Figure 2 is a side elevation of the mechanism shown in Figure lpivotally secured in position by two spaced plates that carry thedamping magnets;

Figure 3 is a top, plan view or" the upper plate showing the soft-ironcylindrical discs employed to concentrate the magnetic fiux linescutting the damper vane; and

2,708,737 Patented May 17, 1955 Figure 4 is a rear elevation view of theassembly shown in Figure 2.

In Figures 1 and 2 of the drawings, the numeral 10 identifies awire-wound, movable coil adapted for rotation about an axis defined by apair of oppositely-disposed pivots 11, 12, that are secured to the coilby means of conventional pivot bases 13, 14, respectively. Secured tothe coil, as by the nut 15, are the pointer 16, and damping vane 1'7,the latter being made of thin, sheet metal having a desired thicknessand electrical resistivity. The pivots of the movable coil areindividually positioned by conventional jewels carried by the jewelscrews 21, 22, that are supported in the spaced plates 23, 24,respectively. Those skilled in this art will understand that the plates23, 24, are secured in fixed, spaced position by suitable posts and thatthe movable coil 10 rotates in a magnetic flux field established by asuitable source of magnetic energy that likewise is secured in fixedposition relative to the coil, whereby the pointer assumes an angularposition, relative to a suitable scale, depending on the reactionbetween the maknetic field flux and the flux resulting from current flowing through the movable coil. Normally, the current is led to themovable coil by means of spiral springs which also provide acounter-torque tending to return the pointer to a fixed positionrelative to the associated scale; such springs, being well known in theart, are not shown in the drawings. In the case of anelectro-dynamometer instrument, the magnetic field is provided by afield coil carried by a suitable laminated yoke, the spaced plates 23,24, being secured to the yoke by suitable studs.

The spaced plates 23, 24, are made of magnetic material, such assoft-iron. As shown in Figures 2 and 4, the lower plate 24 carries threebar magnets 30, 31, and 32 (having upper, polar surfaces spaced arelatively small distance from the vane 17) and the two electromagnets34, 35, the latter comprising coils carried by the soft-iron cores 34,35', respectively. The bar magnets may be provided withreduced-diameter, lower ends that are force-fitted into suitable holesprovided in the plate 24. The soft-iron cores may have ends similarlyformed but being made of malleable material the core ends may extendthrough the holes in the plate and be peaned over to secure the cores infirm, upright position. The upper plate 23 las secured thereto aplurality of soft-iron discs 37- 11, each alined with one of the barmagnets and cores carried by the lower plate. As shown in Figure 3, thediscs 37-41 have reduced-diameter ends extending through holes in theupper plate and such ends being peaned over slightly to secure the partsin place, but any other suitable arrangement may be used to secure. thediscs to the plate.

it will be noted that the outer bar magnets 30, 32, are oriented so thatthe S magnetic pole is adjacent to the vane 17, whereas the inner magnet31 is of reversed polarity. The magnetic flux lines, therefore, emanatefrom the N pole of the bar magnet 31, pass through the vane to thealined soft-iron disc 40, through the upper plate 23 and the soft-irondiscs 39 and 41, to the S poles of the bar magnets 30, 32, and throughthe lower plate 24 to the S pole of the bar magnet 31. Thus, it isapparent that the permanent magnet bars are arranged in seriesrelationship thereby providing a strong mag netic flux field across theair gaps and through the vane. The conducting vane 17 moves through suchmagnetic flux field whereby eddy currents are induced in the vane, themagnitudes of such currents depending on the strength of the flux fieldand the velocity of movement of the vane. The magnetic reaction betweenthe magnetic flux field cut by the vane and that resulting from the eddycurrents is such that it opposes movement of the vane thereby providinga damping action.

The damping efiect provided by the permanent net bars 30, 31, 32,depends upon the strength of the individual bar magnets, the number ofsuch magnets and the relative disposition of their polar ends. I preferto adjust the overall damping effect of the permanent magnets so as toachieve a maximum level of damp-- ing that is slightly below therequired damping factor of the particular mechanism. Such initialdamping effeet is augmented by the electro-rnagnetic damping provided bythe electro-magnets 34, 35, which are energized from a suitable D.-C.source such as the battery 45 shown in Figure 4. Here theelectro-magnets are shown connected in parallel and the strength of themagnetic flux field established at the ends of the cores 34, 35', may becontrolled by a rheostat 46 connected between the elect-ro-magnets andthe battery. Obviously, the elec tro-magnets may be connected in series.Thus, it is sible to control the overall damping effect of theinstrument smoothly, conveniently and accurately from the base levelprovided by the permanent magnet bars to a'higher level that includesthe permanent magnet and clectro-magnetic effects. It is also possibleto provide a wider latitude of damping adjustment by reversing thedirection of the electro-magnetic flux so that it opposes the permanentmagnet flux, whereby the damping characteristics of the mechanism mayhave a controlled range from some desired minimum level to a maximumlevel.

Having now described my invention it is apparent that I provide acombined permanent magnet and electromagnetic damping-effect whereby theactual damping rate of a movable mechanism may be controlled as requiredin a particular instrument.

I claim:

1. A magnetic damping device for cooperation with a damping vane carriedby an instrument movement, said device comprising a pair of spacedplates made of magnetic material, alined bearing members carried by theplates and supporting the movement for rotation therebetween, aplurality of laterally-spaced cylindrical permanent magnets eachhavingan end secured to one of the plates and an end spaced a relatively smalldistance from the vane, a plurality of cylindrical magnetic materialcores each having an end secured to one of the plates and an end spacedat relatively small distance from the vane, electrical coils carried bythe cores, a D.-C. potential source, and an adjustable resistorconnected between each of said coils and the source.

2. The invention as recited in claim 1 including a plurality of magneticmaterial discs carried by the other plate, said discs having ends spacedfrom the vane and alined with the free ends of said permanent magnetbars and cores.

References Cited in the file of this patent UNITED STATES PATENTS

